Nonaqueous electrolyte batteries, such as lithium ion secondary batteries, have been widely used as power sources for portable electronic devices such as laptop computers, mobile phones, digital cameras, and camcorders.
In recent years, with the reduction in size and weight of portable electronic devices, outer casings of nonaqueous electrolyte batteries have been reduced in weight. As outer casing materials, cans made of aluminum have been developed to replace cans made of stainless steel, and further packs formed from an aluminum laminate film have been developed to replace metal cans.
However, packs formed from an aluminum laminate film are soft. Therefore, in a battery using such a pack as the outer casing material (soft pack battery), a gap is likely to be formed between an electrode and a separator due to an external shock or the expansion/contraction of electrodes upon charging and discharging. This results in the problem of reduced cycle life.
In order to solve the problems mentioned above, techniques for enhancing adhesion between electrodes and a separator have been proposed.
As one of such techniques, a separator including a polyolefin microporous membrane and a porous layer made of a polyvinylidene fluoride resin (hereinafter referred to as “PVdF layer”) formed thereon is known (see, e.g., Patent Document 1).
When placed on an electrode and hot-pressed, the separator can adhere well to the electrode via the PVdF layer. As a result, the cycle life of a soft pack battery can be improved.